1. Field of the Invention
The present invention relates to a roller screw structure, and more particularly to a roller screw structure which uses the rolling cone as the rolling assembly.
2. Description of the Prior Art
Conventionally, a roller screw structure 10 (as shown in FIG. 1) comprises a nut 11, a screw 12 and a rolling assembly 13. The rolling assembly 13 includes at the least one constant cross-section roller 131, and the roller 131 rolls between the screw 12 and the nut 11 and moves along the track defined by the nut 11 and the screw 12 in a line-line contact manner. Due to the uniform speed of the self-rotation axes of the roller 131, it will cause the roller to produce different rolling paths at different positions when it is in line-to-line contact with the track. For example, the rolling path of the outer edge of the roller is comparatively long, while the rolling path of the inner edge of the roller is comparatively short. Within the same time period, the different rolling paths of the roller 131 will produce the so-called Speed Difference, and the further rolling action of the roller will cause the idle motion, thus increasing the friction of the partial rolling surface of the roller 131, and accelerating the abrasion of the roller 131 to reduce the service life. Moreover, if the worn roller is not replaced, it will influence the smooth linear movement of the screw 12 and the nut 11.
As shown in FIGS. 2-4, another conventional roller screw uses tapered rollers 30 where the apex of the cones is located on the centerlines of the screw 32 so that the tapered rollers 30 roll without a relative speed difference with respect to their respective rolling surface to prevent slipping, and this conventional roller screw satisfies the relationship:
      tan    ⁢                  ⁢    γ    =                    2            ⁢              r        0                    d      m      
wherein γ is an angle of the self-rotation axis of the roller with respect to the rolling surface of the roller, dm is a pitch circle diameter, r0 is the center rotation radius of roller 30, a lead angle between the screw 32 and the nut 31 is β, and α is an contact angle of the self-rotation axis of the roller with respect to the axis of the screw. This conventional roller screw satisfies the following relations:dm=40r0=4lead=12β=5.45α=80
This conventional roller screw has the following disadvantages:
First, the contact angle is too large so that the roller is unable to bear axial load, which means that this conventional roller screw is incapable of withstanding the vibration caused by screw rotation.
Second, the contact angle is nearly 90 degrees, making it difficult to design the threads 320 of the screw 32, as shown in FIG. 2. The gap between two neighboring threads 320 of the screw 32 must be large enough to maintain the structural rigidity, therefore, the lead of this conventional roller screw has to be very large (nearly two times of the lead of the present invention) and can't adopt the double-thread design.
Third, the contact angle is nearly 90 degrees, so the inner diameter of the nut 31 has to be smaller than the outer diameter of the screw 32, and accordingly the width of the thread 320 of the screw 32 is relatively large, making it complicated and difficult to assemble and manufacture.
The present invention has arisen to mitigate and/or obviate the afore-described disadvantages.